In recent years, along with the rapid development of electronic devices, communication devices, and the like, and the development of miniaturization technology, various portable devices have proliferated. Thus, in consideration of costs, reductions of sizes and weights of devices that are power supplies of these portable devices, and the development of secondary batteries having a high capacity and excellent lifespan characteristics are strongly required.
As such a small, lightweight and high capacity secondary battery, these days, the development of a rocking chair type lithium ion battery in which a lithium intercalation compound that releases lithium ions from between layers is used as a positive electrode material and a carbonaceous material that is typified by graphite that can occlude and release (intercalate) lithium ions between layers of crystal planes during charging and discharging is used as a negative electrode material has progressed, and such batteries have been put into practice and are generally used.
A nonaqueous electrolyte secondary battery in which a lithium compound is used as a negative electrode has a high voltage and a high energy density. Among these, the metal lithium was initially much studied as a negative electrode active material due to its high battery capacity. However, when the metal lithium is used as a negative electrode, since a large amount of dendritic lithium precipitates on surfaces of negative electrode lithium during charging, charging and discharging efficiency decreases, and dendritic lithium grows and short circuiting with a positive electrode easily occurs, or since it is sensitive to heat and impact due to instability of lithium itself, that is, high reactivity, there are still problems for commercialization.
Therefore, instead of the metal lithium, as a negative electrode active material, a carbon-based negative electrode that occludes and releases lithium ions is used.
A carbon-based negative electrode solves various problems of the metal lithium and greatly contributes to the spread of lithium ion batteries. However, as various portable devices are gradually reduced in size and weight and have high performance, increasing a capacity of lithium ion secondary batteries has become an important problem.
A lithium ion secondary battery using a carbon-based negative electrode has an inherently low battery capacity. For example, even in the case of graphite having the highest crystallinity as the carbon used, a theoretical capacity is about 372 mAh/g in a composition of LiC6. This is only about 10% compared to a theoretical capacity of the metal lithium of 3860 mAh/g. Therefore, studies have been actively conducted in order to increase a capacity of a battery by introducing a metal such as lithium again into a negative electrode regardless of existing problems of a metal negative electrode.
As a representative material, use of a material including a metal that can be alloyed with lithium such as Si, Sn, and Al as a main component as a negative electrode active material has been studied. However, a material that can be alloyed with lithium such as Si and Sn has problems that, due to volume expansion during an alloying reaction with lithium, metal material particles are pulverized, and thus contact between metal material particles is less likely to occur, an electrically isolated active material is generated in an electrode, metal material particles are released from the electrode, an internal resistance increases, a capacity decreases, as a result, cycle performance deteriorate, and an electrolyte decomposition reaction due to an enlarged specific surface area becomes serious.
In order to solve problems occurring when such a metal material is used, studies in which two or more types of metal oxide having a relatively lower volume expansion rate than a metal are used as a material of a negative electrode active material are being conducted.
For example, Patent Literature 1 discloses that a material obtained by coating a titanium oxide to a silicon oxide with a silicon concentration gradient is used as a negative electrode active material of a lithium ion secondary battery, and a battery having a high capacity and improved cycle performance is obtained. In the detailed description, it is described that the silicon oxide coated with a titanium oxide is heated under a reducing atmosphere. However, there is no description about an effect on charging and discharging cycle performance, and a heat treatment is performed under an inert atmosphere in examples.
In addition, Patent Literature 2 proposes that a material in which surfaces of silicon oxide particles on which silicon nanoparticles are dispersed are coated with a titanium oxide is used as a negative electrode material for a secondary battery.
In addition, Non-Patent Literature 1 discloses that a material in which a silicon oxide is coated with an anatase type titanium oxide is used as a negative electrode material of a lithium ion battery.
However, although certain improvements in an initial capacity and cycle performance are observed in any known technology in which a material obtained by coating a titanium oxide to a silicon oxide-based compound is used, when charging and discharging are repeatedly performed, a reversibly available capacity gradually decreases, and cycle performance that are available for practical use are not obtained.
In addition, the productivity of a production method for obtaining a negative electrode material is low, and as a result, high costs are required for the technology.